1. Field of the Invention
This invention relates to novel liquid crystal compounds, and more particularly it relates to compounds whose optically active isomers form cholesteric liquid crystals and whose racemates form nematic liquid crystals.
2. Description of the Prior Art
Recently, application of liquid crystals to display elements and practical use of the latter have been greatly advanced, and many liquid crystal compounds have already been found accordingly. However, from the viewpoint of practical uses, i.e. from the standpoint of application of liquid crystal display elements to various fields, liquid crystal compounds further suitable to the respective uses have been desired, and particularly, there have been mostly desired liquid crystal materials which are endurable to long-term uses under severe use conditions. Presently any single compound which satisfies by itself such a requirement has scarcely been found; hence for this purpose it has been necessary to admix a plurality of liquid crystal compounds which are stable and varied in the liquid crystal temperature range to obtain liquid crystal compositions suitable to the respective use purposes. The present invention is directed to novel liquid crystal compounds which are effectively usable for such an object.
Cholesteric compounds as one kind of liquid crystal compounds have generally raised mostly a problem of stability; among them, however, chemically stable cholesteric compounds have already been known. Examples thereof are as follows: ##STR2## Cholesteric temperature: 163.degree..about.186.degree. C. Melting point: 120.degree. C. (This compound forms a smectic liquid crystal between 120.degree. C. and 163.degree. C.).
(G. W. Gray; J. de Physique, 36, Cl 337 (1975)). ##STR3## Cholesteric temperature: -54.degree..about.-30.degree. C. Melting point: 4.degree. C. PA1 (G. W. Gray, D. G. Donnell; Electronic Letters, 11 (23), 556 (1975)). ##STR4## Cholesteric temperature: -22.degree..about.-14.degree. C. Melting point: 9.degree. C. PA1 (The literature is the same as that of (2)). ##STR5## Cholesteric temperature: -10.degree..about.-20.degree. C. Melting point: 28.degree. C. PA1 (The literature is the same as that of (2).). ##STR6## Cholestric temperature: 24.degree..about.30.degree. C. (Japanese patent application laid-open No. Sho 51-78791/1976)
However, the respective cholesteric temperatures of these compounds are either in a very high temperature range or in a very low temperature range, and further these ranges are narrow; hence these compounds are unfavorable as constituting components of liquid crystal mixtures having a sufficiently broad cholesteric temperature range as liquid crystal substances which can be employed for various usual uses. Thus, substances which exhibit a cholesteric phase in a more suitable temperature range are required.
For attaining the above-mentioned object, the present inventors previously searched for suitable cholesteric liquid crystal compounds and applied for patent with regard to some of them (Japanese patent application Nos. Sho 51-95,874/1976, Sho 51-106,384/1976, Sho 51-116,849/1976, Sho 52-25,308/1977, Sho 52-31,734/1977, etc.), and further have searched for such compounds, and as a result have found the compounds of the present invention which are stable as in the case of the above-mentioned compounds and yet have broader and preferable liquid crystal temperature ranges.